User space is that portion of system memory in which user processes run. This contrasts with kernel space, which is that portion of memory in which the kernel executes and provides its services.
The contents of memory, which consists of dedicated RAM (random access memory) VLSI (very large scale integrated circuit) semiconductor chips, can be accessed (i.e., read and written to) at extremely high speeds but are retained only temporarily (i.e., while in use or, at most, while the power supply remains on). This contrasts with storage (e.g., disk drives), which has much slower access speeds but whose contents are retained after the power is turned off and which usually has a far greater capacity.
A process is an executing (i.e., running) instance of a program. User processes are instances of all programs other than the kernel (i.e., utility and application programs). When a program is to be run, it is copied from storage into user space so that it can be accessed at high speed by the CPU (central processing unit).
The kernel is a program that constitutes the central core of a computer operating system. It is not a process, but rather a controller of processes, and it has complete control over everything that occurs on the system. This includes managing individual user processes within user space and preventing them from interfering with each other.
The division of system memory in Unix-like operating systems into user space and kernel space plays an important role in maintaining system stability and security.
